Beside the methane emanating from technical installations and by a natural way from the soil, carbon dioxide is the most important greenhouse gas. It destructs the ozone belt which is protecting life on earth from to intensive ultraviolet radiation.
Since more than a decade technicians and scientists are seeking for methods and means in order to eliminate the noxious CO2 from waste gases and to store it in the form of an insoluble chemical compound or outside of the atmosphere of the earth. There are technically feasible proposals (partly tested in pilot plants) in order to separate the CO2 from combustion or other waste gases, to concentrate it (by liquefing), to use it industrially or to store it underground (in aquifers, deep sea or to stimulate poorly producing oil reservoirs). One of the practical ways to concentrate the CO2 consists of reintroducing the combustion CO2 by a secondary cycle in the combustion process after having made the combustion-air nitrogen-free by catalytic and/or cryogenic separation. Instead of the use in neighbouring chemical plants, the binding of CO2 on sea weed as well as the bacterial fermentation of the CO2 in special reactors have been technically examined in pilot plants. These latter processes need either large storing volumes for the CO2 resp. CH4 produced or an absolutely constant operation of the CO2 producing installations, because these binding resp. fermentation processes are not flexible. Specially thermal power plants demand such flexibility in order to cover daily, weekly and seasonal peaks.
The present invention permits such flexible operation. The invention comprises the pumping of the separated, purified, liquefied and dryed CO2 in a pipeline to a nearby aquifer, a natural gas reservoir or an oil bearing structure and injecting the CO2 into it and treating the structures in the first two cases previously with bacteria cultures (and ev. suitable substrate). In the third case the methanogenic bacteria and the substrate will be already naturally present so that the conversion of the CO2 to CH4 may immediately begin. For that purpose the following conditions have to be naturally present or artificially created (see bibliography in the annex):
1. There has to be enough H2 for reduction of the CO2 (or as electron donor) in the structure (underground reservoir), either
in free form produced by metamorphic rock in the depth, or
by bacterial direct transmission from organic H2 containing substrate, or
by bacterial splitting off of the pore water, or
by artificial feeding of H2 in pure form of ammonia (NH3) which forms in the structure (underground reservoir), in contact with the pore water, partly ammoniumhydroxide NH4OH and with CO2 urea (H2N)2CO which, by bacterial aided hydrolyzing, splits off free H2, or
e.g. by artificial feeding of formiates NaHCO2 or Ca(HCO2)2.
2. The sulfate content of the sediment in the structure (underground reservoir) has to be either
enough poor that the sulfate no more can selectively bind the H2 necessary for the CO2 reduction, and/or
no H2S or other sulfide may act as methanogenic poison, or
the sulfate competing with the CO2 reduction may be artificially blocked by an inhibitor (e.g. NA-molybdate or fluorlactate), or
by feeding H2 in to the structure (underground reservoir) enough long before the CO2 injection, so that the sulfate will be sufficiently reduced.
3. There must be present in the sediment a methanogenic bacterial population large enough (e.g. methanococcus, methanobacterium formicicum, methanobacterium thermoautotrophicum, methanosarcina barkerii, or other photosynthetic bacteria) which is able,
to split off free H2 from the pore water or from H2S, and
to reduce the CO2 to CH4 sufficiently,
eventually by addition of catalysators like e.g. palladium,
adding, if necessary, the needed bacterial population and/or the substrate artificially.
The main condition to realize the CO2 conversion is a drilling (in a aquifer, a natural gas reservoir or an oil bearing formation) across the gas or oil containing structure till in the aquatic phase thereunder, with cores from the interesting stratums and analyzing these cores in the laboratory.
The main aim is finding (or not) the necessary bacteria, the substrate whereon they live, the chemical composition of all sediment parts, the presence of H2, sulfate as well as the absence of Naxe2x80x94AL-silicates. With these results it will be further established under which conditions the found (or added) bacteria and the substrate are able to produce CH4 from CO2 and how much.
By adding other substrates (e.g. acetate, methanol, methylamine, dihydronicotinamide, dihydro-5-diazaflavine, 2-mercaptoethane-sulfonic acid and like) it will be established if the CO2 reduction to CH4 is intensified resp. accelerated. With a given substrate the addition of CO2, in the presence of H2, increases the CH4 production normally by more than one magnitude. The same is the result when H2 is added in presence of CO2. Consequently, both gases have to be present together for ensuring an optimal CH4 production. In this connection it may be noted that the 4 H atoms needed for the CH4 do not originate from the added H2 gas (or NH3), but from the pore water. The H2 gas serves only as electron donor. All research work hitherto existing shows that bacteria and substrate are only a transition station for the H2 necessary for CO2 reduction. Consequently, during the laboratory experiments the consumption of bacteria and substrate,as well naturally present as also artificially added matter, has to be established. Concerning the practical use of the invention, a consumption of bacteria and/or substrate to high could dictate an economic limit.
As soon as the different questions are resolved by the laboratory experiments, the same tests are following under in situ conditions of pressure and temperature (still in the laboratory). This clarifying by steps conducts to the optimum method which is than tryed out in a pilot test 1:1 after a second drilling (observation drilling) in relative vicinity to the first drilling. If there is a flow direction of the pore water, the testing has to be made on the low side and the CO2 addition on the high side of the flow. An increasing CH4 content signifies a positive conversion, which is confirmed by an also increasing H2 content.
This is specially the case with the addition of ammonia gas instead of pure H2. The basic chemical transformation according to the formula CO2+4 H2=CH4+2 H2O becomes with ammonia in molecular writing 2 CO2+6 NH3=2 CH4+3 N2+4 H2O+H2, what means that the transformation yields a H2 excess which passes over in the next step, so that, in constant operation, will be an equilibrium CO2: NH3 of approx. 1:2,7. But this will only be the case if the H2 procurement is not aided by bacterial assistance. Such bacterial assistance liberates practically always supplemental H2 from organic substrates or from pore water, eventually supported by H2 from metamorphic rock, what reduces the CO2/NH3 proportion from 2,7 to 1 and less.
The CO2 reduction produces in every case additional pore water. The use of NH3 as H2 donor has the special disadvantage of producing the double quantity of pore water compared to the use of pure H2 and that 40% of the NH3 gas remain as N2 (ballast) in the structure and appear occasionally as pollution in the produced CH4 and demand a separation (purification). Economically it could be better to inject pure H2 under high pressure as emulsion in the liquid CO2 (with or without emulgators). At the maximum possible proportion of volume CO2:H2 of 1:4 the emulsion becomes foamy and at smaller H2 requirements and higher pressure the H2 is broken up in small gas bubbles in the liquid CO2.
The liquid medium CO2 covers than the inner surface of the steel tube and not the H2 gas and forms a protection layer having itself a soluted proportion H2:CO2 (volume) of approx. 1:100 (at the normally present conditions of pressure and temperature) and protects in such manner the steel from brittleness by direct hydrogen attack. For this protection it is essential that there is a turbulent flow in order to maintain the CO2/H2 emulsion during injection in the structure. Moreover the structure temperature (approx. 80xc2x0 C.) is not high enough to really cause hydrogen brittleness (over 300xc2x0 C.). Because of the difficulty to control long pipelines, the H2 should be produced in the field at the injection point. But there is no danger for an auto-reduction of the CO2 to CH4 within the CO2/H2 mix in the injection tube at the existing temperatures.
If the CO2 conversion takes place in a aquifer, the addition of H2, bacteria and/or substrate is unavoidable, but may be avoidable in a natural gas reservoir created by gas migration and is most probably not necessary in oil and gas bearing (generating) formations. The eventual additions beside the CO2 (NH3, H2, bacteria, substrates, catalysators, inhibitors) shall not pollute the groundwater of aquifers being situated above the deep lying brack waters. For these latter the pollution aspect is of minor significance.